1. Field of the Invention
This invention relates to a method for the production of a silicon single crystal of a large diameter and a great weight by Czochralski method, and more particularly to a technique which especially, controls the element which dissolves into the melt from the crucible and impurity which becomes entrapped to inside of the crystal, while the stabilization of the melt flow is attempted.
2. Description of the Related Art
Recently, a demand of the silicon wafer in which the caliber is bigger in response to area expansion of semiconductor device heightens. The crystal weight with the necessity exceeds 150 kg in the crystal for the wafer of the 200 mm size, and exceeds 500 kg in the crystal for the wafer of 300 mm size. The production technique of such crystal is inevitably limited to Czochralski method, and the scale-up of the process will be done using a crucible of an unprecedented large size.
However, the convection of silicon melt which is generated in crucible of the Czochralski method is stronger, as crucible diameter and melt depth are increased, and again, the method is accompanied by large turbulence. The turbulence of the melt convection exerts advance effects on the growth of crystal for the following three reasons.
Firstly, the disturbance of the convection brings an irregular temperature fluctuation along the course of time on the growth face of crystal and, at the same time, disrupts the axial symmetry of the melt temperature distribution. As a result, the thermal balance on the crystal growth interface is disturbed and the crystal growth as a single crystal is inhibited.
Secondly, the fortification of the convection promotes the transport of impurities from the bottom of a deteriorated quartz crucible and the impurities, on contacting the crystal in growth, make the crystal polycrystalline.
Thirdly, the disturbance of the convection gives rise to uneven concentrations of oxygen and dopant in the silicon melt. Even when a crystal free from dislocation is successfully grown, this disturbance causes wafers sliced off from the crystal to suffer from uneven quality in terms of oxygen concentration and magnitude of resistance.
As a way of curbing the adverse effects of the convection of the melt in the crucible of the Czochralski method, JP-B-02-12920 proposes a method for repressing the flow of the silicon melt by applying a static magnetic field of the cups to the melt. This method is claimed to impart an improved shape to the crystal by repressing the disturbance of the convection by virtue of the Lorentz force and prevent the crystal from pollution with the impurities transported from the crucible by repressing the thermal convection occurring in the radial direction. This publication barely cites a working example purporting manifestation of the effect thereof in repressing the amount of lattice defects in a GaAs crystal.
JP-A-01-282,185 and JP-B-08-18,898 show that even in a silicon single crystal, the application of a cusped magnetic field controls the oxygen concentration in the crystal by repressing the melt convection.
It has been meanwhile demonstrated that the repression of the convection in the silicon melt by the application of a static magnetic field results in impairing the homogenization effect of crystal quality by the crystal rotation. Japanese Patent No. 2,706,165 proposes a contrivance for repressing the strength of the vertical component of the magnetic field directly under a crystal.
It is not safe to conclude that the mechanism for repressing the convection by the application of a cusped magnetic field operates in entirely the same manner on silicon melt of a large amount exceeding some hundreds of kg. This assertion is based on the following reasons.
Firstly, the amount of silicon melt in the crucible used in working examples cited in JP-B-08-18,898, Japanese Patent No. 2,706,165, and JP-01-282,185 fall short of 1 kg in the first two cases and 34 kg in the last case. These amounts are smaller by not less than one decimal position than that of melt used in manufacturing the crystal for 200 mm or 300 mm size wafer.
Secondly, the extents of the effect of applying the magnetic field have not been clearly quantified, when changing the magnetic field strength and the container size, and varying the physical constants of raw material melt such as viscosity, coefficient of cubic expansion, electric conductivity, and density.
Thirdly, it has been discovered by the actual experiment on the temperature measurement of silicon melt produced by the Czokralski method that in a large crucible, the axial symmetry of the convection in the crucible is drastically disturbed to the extent of entailing the state of baroclinic wave (Kishida et al.: J. Crystal Growth (1992) and Watanabe et al.: J. Crystal Growth (1993)), the state of geostrophic turbulence (JP-A-09-255,790), and the state of hard turbulence and soft turbulence (Togawa et al.: J. Crystal Growth (1996), and JP-A-08-259,379). As a result, the form of the flow of the silicon melt itself to be controlled is completely different from those which are illustrated in the technics of applying the cusped magnetic field mentioned above.